NUSAYBIN, Turkey, March 20 (Reuters) - U.N. aid trucks crossed from Turkey into Syria for the first time on Thursday, in a move relief officials hope will pave the way for greater humanitarian access to civilians hardest hit by three years of war.

But with the convoy heading to a region controlled by forces loyal to President Bashar al-Assad, doubts remained whether those in rebel-held areas most urgently in need would benefit.

The trucks rolled through the largely deserted frontier at the Nusaybin border post taking food supplies, bedding and medicine towards the ethnic Kurdish city of Qamishli in Syria.

"It is the first time in three years of this brutal conflict that the U.N. has been able to carry aid across into Syria from Turkey," Nigel Fisher, the United Nations' regional humanitarian coordinator, said in a statement.

The convoy became possible after the U.N. Security Council last month unanimously called on Syrian authorities and rebels to allow humanitarian supplies across front lines and borders by the most direct routes, to reach an estimated 9.3 million people in need.

Western aid officials say the joint effort by agencies including the World Food Programme (WFP), children's agency UNICEF and the World Health Organization (WHO) is a step towards alleviating suffering in a conflict that has already claimed the lives of 140,000 people, although they privately express concerns that the worst affected will still not be helped.

"THIS AID IS USELESS"

Small crowds gathered at the Nusaybin frontier as the convoy crossed, containing supplies for more than 50,000 people.

A Syrian government flag was fluttering on the other side of the border as the trucks got under way, with the aid due to be distributed by partners of the regime as stipulated by Damascus, sources said.

A spokeswoman for WFP said that the organisation was working out a delivery plan "aiming to get the food to both sides of the conflict".

Abeer Etefa said that up until now WFP had resorted to hugely expensive and limited air drops to reach 200,000 needy people in Hasakah governorate, where the trucks are headed.

"We hope that in the future this becomes a way to get food to desperate families (there)," she told Reuters.

But one resident of Qamishli said the government had no intention of helping the population in general, and would give supplies only to their own side.

"Qamishli is under the control of Assad's soldiers," said 33-year old Adnan, who didn't wish to give his surname.

"They give to the people who they like and they give to the soldiers' families. This aid is useless."

U.N. Secretary General Ban Ki Moon is expected to report on implementation of last month's resolution within a month of aid starting to flow, with the option to recommend "further steps in the case of non-compliance."

However diplomats have said Russia would almost certainly continue to block any sanctions against Damascus. (Additional reporting by Dasha Afanasieva in Gaziantep; Writing by Jonny Hogg; Editing by Nick Tattersall/Ruth Pitchford)